Magnetically-levitated blood pump with optimization method enabling miniaturization

ABSTRACT

A magnetically-levitated blood pump with an optimization method that enables miniaturization and supercritical operation. The blood pump includes an optimized annular blood gap that increases blood flow and also provides a reduction in bearing stiffness among the permanent magnet bearings. Sensors are configured and placed optimally to provide space savings for the motor and magnet sections of the blood pump. Rotor mass is increased by providing permanent magnet placement deep within the rotor enabled by a draw rod configuration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/054,903, filed on May 27, 2011 which claims the benefit of priorityof U.S. Provisional Patent Application No. 61/100,655, filed Sep. 26,2008, the disclosures of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contractnumber HHSN262800448192C awarded by the National Institutes of Health.

FIELD OF THE INVENTION

The subject matter of the present disclosure generally relates to a pumphaving a magnetically-levitated rotor. More particularly, it relates toa blood pump used as a ventricular assist device (VAD) for providingcardiac assist. The invention enables miniaturization via supercriticaloperation of the spinning pump rotor. With this enhancedminiaturization, the VAD can be implanted in a less invasive surgicalprocedure or can be used with infants.

BACKGROUND OF THE INVENTION

Roughly 700,000 patients die from heart disease in the U.S. each yearand 35,000 to 70,000 of these could benefit from mechanical circulatorysupport or a heart transplant. However, only about 2,500 transplanthearts become available each year. This translates to a profound needfor a reliable mechanical blood pump to serve as a cardiac assist deviceor artificial heart.

Several prior-art devices attempt to solve this problem. Indeed,numerous embodiments of blood pumps exist, but are subject tosignificant operational problems. Several such prior-art pumps arediscussed herein below.

In U.S. Pat. No. 4,688,998 issued to Olsen et al., a motor stator isdisclosed that consists of C-shaped rings. The rings substantiallyincrease the diameter of the pump contrary to the anatomical requirementof small size and weight.

In U.S. Pat. Nos. 4,763,032, 4,944,748, 5,078,741, 5,326,344, and5,385,581, all issued to Bramm et al., a device is disclosed thatrequires two inflow channels, which increase the total blood-wettedsurface. Among other things, this large contact area between artificialmaterials and the blood increases immune system response to the pump aswell as the probability of thromboembolism. Further, connecting the twoinlets of this particular pump to the heart is complex and requiresadditional tubing. Thus, anatomical interference of such pumps withnatural organs and structures is increased.

In FIG. 31 of U.S. Pat. No. 4,944,748, there is also disclosed anaxial-flow magnetically-levitated blood pump. However, a pump thatoffers minimal pump volume for a given flow is not disclosed. Inparticular, the embodiment of FIG. 31 depicts a narrow gap for bloodflow between the rotor and the housing. While this is advantageous formagnetic bearing stiffness, it does not lead to a miniaturized pump.FIG. 31 of the '748 patent suggests a ratio of gap to rotor diameter ofroughly 1/30 and there is no disclosure of a process for optimallychoosing this gap. Nor is there a disclosure of a method of choosing therotor speed, rotor inertias, and magnetic bearing stiffness incombination to achieve a miniaturized pump.

In U.S. Pat. No. 5,112,202 issued to Oshima et al., a device isdisclosed in the form of a centrifugal pump that utilizes a magneticcoupling with mechanical bearings subject to wear. This pump is notsuitable for long-term implantation as the bearings will eventually faildue to wear.

In U.S. Pat. Nos. 5,195,877 and 5,470,208 issued to Kletschka, a deviceis disclosed that requires two inflow channels, which increases thetotal blood-wetted surface. This large contact area between artificialmaterials and the blood increases immune system response to the pump.The large surface area also increases the probability ofthromboembolism. Further, connecting the two inlets of the pump to theheart is complex and requires additional tubing. Thus, anatomicalinterference of the pump with natural organs and structures isincreased.

In U.S. Pat. No. 5,443,503 issued to Yamane, a pump device is disclosedthat has a jewel bearing. Such bearings are subject to wear in along-term implant. Further, the jewel bearing is a point of blood stasisand is subject to clotting and may lead to thromboembolism. Washoutholes are provided to counter such blood stasis, but such washout holesthemselves increase the total blood-wetted surface and thus likelynegating any benefit.

In U.S. Pat. No. 5,507,629 issued to Jarvik, a device is disclosed thatincludes a mechanical bearing in the form of a jewel bearing which is apoint of blood flow stasis. The blood stasis point is a location ofthrombus formation and a source of thromboembolism. Other embodiments ofthis invention levitate the rotor using only passive magnetic bearingsthat are inherently unstable, especially during the requisite high-speedrotor rotation. Unstable rotors can contact the pump housing andpotentially stop the blood flow.

In U.S. Pat. Nos. 5,695,471 and 5,840,070, both issued to Wampler, ablood pump is disclosed. Wampler '471 is similar to the device of Jarvik'629 in that there is a stasis point at the jewel bearing. The stasispoint is a site of thrombus formation and a source of thromboembolism.Further, the jewel bearing will eventually wear out and the impellerwill cease to rotate. Wampler '070 uses a hydrodynamic thrust bearing.Such a bearing is highly inappropriate for use within blood processingbecause such bearings can damage the blood via the high mechanical shearthat is inherent to such bearings.

In U.S. Pat. No. 5,725,357 issued to Nakazeki, a device is disclosed inthe form of a pump that contains a motor with mechanical bearingssubject to wear. Such a device is not suitable for a long-term implantas the mechanical bearings will eventually fail and cause the pump tostop working.

In U.S. Pat. No. 5,928,131 issued to Prem, an elongated pump isdisclosed that exposes blood to large regions of foreign material andincreases the likelihood of blood damage and thrombus formation. Thereis also a large region of high blood shear. Blood shear causes blooddamage and can trigger undesirable clotting mechanisms in the body.Further, no manner is disclosed for choosing the blood gap, rotorinertias, rotor speed range, and magnetic bearing stiffness to achieve aminiaturized pump.

In U.S. Pat. No. 6,761,532 issued to Capone et al., an axial-flowmagnetically-levitated blood pump is disclosed. However, no means forminiaturizing the pump is disclosed. In particular, the embodiment ofFIG. 2 of this patent depicts a narrow gap for blood flow between therotor and the housing. While this is advantageous for magnetic bearingstiffness, it does not lead to a miniaturized pump. FIG. 2 of the '532patent suggests a ratio of gap to rotor diameter of roughly 1/30 andthere is no disclosure of a means for optimally choosing this gap nor isthere a disclosure of choosing the rotor speed, rotor inertias, andmagnetic bearing stiffness in combination to achieve a miniaturizedpump. Moreover, the '532 patent places axial position sensor in such away as to significantly elongate the pump.

Still further, several Ventricular Assist Device (VAD) systems have beendeveloped over the years for bridge to implant, destination therapy, andas a bridge to recovery. A general understanding of such devices can begained by reviewing the cardiac assist products of World Heart Inc. ofSalt Lake City, Utah or Thoratec Corporation of Pleasanton, Calif.

From the discussion above, it becomes readily apparent that existingdevices on the market are overly complex, prone to mechanical failure,promote thromboembolism and strokes, and otherwise suffer fromshortcomings related to their ineffective designs. Moreover, none ofthese designs offer a combination of magnetic levitation and small size.None disclose the use of supercritical operation for miniaturization,nor do they disclose the design of rotor-to-housing gaps in combinationwith rotor mass, speed ranges, and bearing stiffnesses to achievesupercritical operation and small size. Further, none disclose a cableattachment and internal interconnection space supportingminiaturization.

Accordingly, it is desirable to provide for a new and improved,effective rotary blood pump suitable for long-term implantation intohumans for artificial circulatory support. What is needed is such ablood pump that is highly reliable. What is also needed is such a bloodpump that meets anatomical requirements with a very compact physicaldesign. What is further needed is such a blood pump that minimizesblood-wetted surface area. Still, what is needed is such a blood pumpthat minimizes deleterious effects on blood and its circulatory system,the immune system, and other related biological functions. What is alsoneeded is such a blood pump that is not only resilient to everydayaccelerations and bodily movements, but also includes stable rotordynamics, a high motor efficiency, high fluid efficiency, low powerconsumption for levitation, low vibration, low manufacturing costs, andincreased convenience to the patient. Still further, what is needed is ablood pump that overcomes at least some of the disadvantages of theprior art while providing new and useful features.

SUMMARY OF THE INVENTION

The present invention provides a magnetically-levitated (maglev) bloodpump suitable for use as a Ventricular Assist Device (VAD) thatovercomes the deficiencies of the prior art.

In general, maglev blood pumps have a combination of feedback-controlled(or “active”) magnetic bearings and permanent magnet (PM) magneticbearings. PM magnetic bearings are inexpensive, energy-efficient, andlow-cost but have relatively low stiffness compared to hydrodynamicbearings or active magnetic bearings. A challenge with the design ofmaglev blood pumps employing PM magnetic bearings for radial bearings isthat resonances can occur in the range of operating speeds of the rotorused for pumping. The essentially rigid rotor mass, gyroscopic forces,and spring properties of the magnetic bearings work together to createthese mechanical resonances of the rotor that, in general, depend on therotor speed. A rotor speed at which a resonance is excited by imbalancesin the rotor is called a “critical speed.” When the rotor speed is aboveat least one critical speed, the operation is referred to as“supercritical operation.” When the rotor speed is below all thecritical speeds, the operation is referred to “subcritical operation.”

It is desirable to avoid critical speeds in the normal operating speedranges to avoid vibration and possible touchdown of the rotor to thehousing. The present invention pertains to component sizing such thatsupercritical operation is consistent with effective pumping andminiaturization of the pump. In particular, the present invention uses alarge gap between the housing and rotor. The large gap in itselfincreases the flow. Moreover, the present invention uses the larger gapto lower the stiffness of the PM magnetic bearings and hence lower thecritical speeds. As a consequence, the entire range of desired pumpspeeds lies above the rotor critical speeds. Advantageously, high rotorspeeds in combination with the large gap allow for high flows in a smallpump according to the present invention. Because the present inventionis designed for supercritical operation, increasing the rotor mass ispossible while simultaneously increasing the speed range of operation atthe low end. Thus, the proportion of motor mass and PM magnet mass inthe rotor can be made larger (e.g., by using the space in the commonlyhollow rotor designs) and the mass in the housing can be smaller.Accordingly, the housing size is reduced and the overall pump size isreduced.

In one implementation, the invention is directed at supporting thesmallest of human patients as a bridge to transplant. In yet anotherimplementation, the invention can be adapted as a minimally invasivesystem for providing cardiopulmonary support for adults.

The present invention has multiple implementations and applications. Themaglev axial mixed-flow ventricular assist device according to thepresent invention is directed towards having the following benefits:fully maglev system without any contacting parts during normaloperation; miniaturization though large rotor-to-housing gap andsupercritical operation; streamlined blood flow with minimal blooddamage and thrombus formation; miniaturization for use in adults,children, and infants; miniaturization to enable minimally-invasiveimplantation; high reliability due to negligible wear; and minimizedpower requirement due to large flow gaps.

In a first aspect of the invention, there is provided amagnetically-levitated blood pump, the blood pump including: an inflowend providing for entry of blood; an outflow end providing for exit ofthe blood; a stator oriented in axial alignment with, and locatedbetween, the inflow end and the outflow end, and including at least onestator permanent magnet and a motor coil; a rotor centered within thestator, and including a plurality of rotor permanent magnets; at leastone permanent magnet bearing formed from a first portion of the at leastone stator permanent magnet and a first corresponding portion of therotor permanent magnets; a motor magnet for interaction with the motorcoil, and formed from a second corresponding portion of the rotorpermanent magnets; and an annular blood gap formed between an outermostsurface of the rotor and an innermost surface of the stator, where aratio of the annular blood gap to rotor diameter is greater than 1/10.

In a second aspect of the invention, there is provided amagnetically-levitated blood pump, the blood pump including: an inflowend providing for entry of blood; an outflow end providing for exit ofthe blood; a stator oriented in axial alignment with, and locatedbetween, the inflow end and the outflow end, and including a pluralityof stator permanent magnets, at least one voice coil, and a motor coil;a rotor centered within the stator, and including a plurality of rotorpermanent magnets; a first permanent magnet bearing arranged near theinflow end, and formed from a first portion of the stator permanentmagnets and a first corresponding portion of the rotor permanentmagnets; a second permanent magnet bearing arranged near the outflowend, and formed from a second portion of the stator permanent magnetsand a second corresponding portion of the rotor permanent magnets; amotor magnet for interaction with the motor coil, and formed from athird corresponding portion of the rotor permanent magnets; and a fourthcorresponding portion of the rotor permanent magnets for interactionwith the at least one coil, the at least one voice coil configured toadditionally interact with the first corresponding portion of the rotorpermanent magnets.

In a third aspect of the invention, there is provided a method ofoptimizing a magnetically-levitated blood pump, the method including:providing a rotor for the blood pump with a plurality of permanentmagnet rings configured to increase rotor mass; providing statorpermanent magnets located on a stator, the stator permanent magnetscorresponding to a portion of the plurality of permanent magnet rings,the stator permanent magnets and the plurality of permanent magnet ringsforming magnetic bearings having a reduced stiffness; and configuringthe rotor and the stator to enable the reduced stiffness.

In a fourth aspect of the invention, there is provided a method ofoptimizing a magnetically-levitated blood pump for supercriticaloperation, the method including: providing a stator oriented in axialalignment with, and located between, an inflow end providing for entryof blood and an outflow end providing for exit of the blood, andincluding a plurality of stator permanent magnets, at least one voicecoil, and a motor coil; providing a rotor centered within the stator,including a plurality of rotor permanent magnets, and configured toprovide a ratio of rotor diameter to an outflow end diameter of lessthan 2; and providing a first permanent magnet bearing and a secondpermanent magnet bearing, the first permanent magnet bearing arrangednear the inflow end and formed from a first portion of the statorpermanent magnets and a first corresponding portion of the rotorpermanent magnets, the second permanent magnet bearing arranged near theoutflow end and formed from a second portion of the stator permanentmagnets and a second corresponding portion of the rotor permanentmagnets, and the first and second permanent magnet bearing having areduced stiffness enabled by a gap configuration formed between therotor and the stator.

Other advantages and benefits may be possible, and it is not necessaryto achieve all or any of these benefits or advantages in order topractice the invention as claimed. Therefore, nothing in the forgoingdescription of the possible or exemplary advantages and benefits can orshould be taken as limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, both as to organization and methods of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description, taken inconjunction with the accompanying drawings in which:

FIGS. 1 through 4 show, respectively, a perspective view, top view,front-end view, side view, and back view of the preferred embodiment ofthe present invention in a fully assembled configuration.

FIG. 5 is a lengthwise, cross-sectional top view taken alongcross-section line A-A of FIG. 3.

FIG. 6 is a lengthwise, cross-sectional side view taken alongcross-section line B-B of FIG. 3.

FIG. 7 is an enlarged detail view of the preferred embodimentcorresponding to detail circle labeled C in FIG. 5.

FIG. 8 is an enlarged detail view of the preferred embodimentcorresponding to detail circle labeled D in FIG. 5.

FIG. 9 is a side view of the stator according to the preferredembodiment with outer housings and cannula adaptors removed.

FIG. 10 is a cross-sectional view taken along cross-section line E-E inFIG. 9.

FIG. 11 is a cross-sectional view taken along cross-section line F-F inFIG. 9.

DETAILED DESCRIPTION

As utilized herein, terms such as “about”, “approximately”,“substantially” and “near” are intended to allow some leeway inmathematical exactness to account for tolerances that are acceptable inthe trade.

Before explaining the present invention in detail, it should be notedthat the invention is not limited in its application or use to thedetails of construction and arrangement of parts illustrated in theaccompanying drawings and description. The illustrative embodiment ofthe invention may be implemented or incorporated in other embodiments,with or without variations and modifications, and may be practiced orcarried out in various ways without straying from the intended scope ofthe present invention. Furthermore, unless otherwise indicated, theterms and expressions employed herein have been chosen for the purposeof describing the illustrative embodiment of the present invention andshould not be construed as limiting the intended scope of the presentinvention. Further, it is understood that any one or more of thefollowing-described embodiments, expressions of embodiments, examples,methods, . . . etc. can be combined with any one or more of the otherfollowing—described embodiments, expressions or embodiments, examples,methods, . . . etc.

FIG. 1 shows a perspective view of the blood pump 20 in accordance withthe preferred embodiment of the present invention. As seen from theoutside, the blood pump is an axial system with an inflow end 21 andoutflow end 22. Conduit adaptors 44 a and 44 b, fastened respectively byadaptor nuts 47 a and 47 b, are provided at both ends for attachment ofthe blood pump 20 to an inflow cannula at the inflow end 21 and anoutflow cannula at the outflow end 22 (cannulas not shown). The bloodpump 20 is encased by an exterior housing comprising an exterior housingcover 23 and a housing end cap 26. The exterior housing has a dome 24 toaccommodate an axially-oriented cable feed through assembly 50. The domestructure 24 also accommodates the electrical interconnections to theblood pump 20, enables anatomic fit, simplified surgical procedures, andoverall miniaturization. It should be readily understood within thecontext of the invention that the exterior housing cover 23 couldoptionally be reversed with the housing end cap 26 and cable feedthrough assembly 50 oriented in reverse on the inflow end 21.

Also shown in FIG. 1 is the inflow lumen 37 through which blood flows,the cable assembly 53 for the electrical interconnections to theinternal workings of the blood pump 20. That is to say, such electricalinterconnections carry sensor signals from the levitation system, andpower to the motor and levitation system. The cable and its terminationswhich form the cable assembly 53 are protected by an elastic strainrelief boot 54. A retention ring 55 clamps the strain relief boot 54 toan underlying connector structure.

FIG. 2 is a top view of the blood pump 20 showing features as describedabove in the discussion of FIG. 1. Here, the inflow end 21 is evidentwhile the outflow end is hidden from view beneath the cable feed throughassembly 50. Likewise, FIG. 3 is a front-end view of the blood pump 20and FIG. 4 is a side view of the blood pump 20 each showing features asdescribed is the discussion of FIG. 1. It should be noted that the slimprofile of the present inventive blood pump 20 as clearly seen by FIGS.2 through 4 assures anatomic fit and simplified surgical procedures. Theasymmetric shape of the pump housing cover 23 visible in FIGS. 1, 3, and4 enables minimal use of space for accommodation of internal componentsthus minimizing the size of the system.

FIG. 5 is a section view of the blood pump 20 corresponding to thecross-section taken across line A-A indicated in FIG. 3. In addition tothe external features shown in FIG. 1, the internal features of theblood pump 20 are shown in FIG. 5 and described as follows. As shown,blood flow is designed to flow in the direction from inflow lumen 37 tooutflow lumen 38 and thereby respectively guide the blood flow into andout of pump. The rotor assembly 60 spins and pumps blood via attachedimpeller blades 62. Stationary stator blades 102 direct the flow at theoutlet end 22 of the blood pump 20. The rotor assembly 60 is rotated viaa 4-pole motor assembly 124 forming stator components including motoriron 125, motor windings 126, and potting material 127 and rotorcomponents including motor magnets 70. An advantage of a 4-pole motordesign is that the rotor magnets collectively have no net dipole moment(up to manufacturing variations) so that there is minimal residualtorques due to interactions with fields created by the PM statormagnets. 2-pole motors do not have this property, while 2N-pole motor dohave this property when the integer N is greater than or equal to 2.Motor rotor magnets which collectively have nearly zero net dipolemoment provide the benefit of reduced excitation of unwanted rotormotion at critical speeds.

FIG. 5 also shows radial support to the rotor assembly provided by foreand aft PM magnetic bearings. The fore PM magnetic bearing includesrotor PM rings 68 a and 68 b and corresponding stator PM rings 121 a and121 b. Similarly, the aft PM magnetic bearing includes rotor PM rings 68c and 68 d and corresponding stator PM rings 121 c and 121 d. Themagnetization directions of the various permanent magnet components areindicated with arrows. Although two PM rings are shown in both the foreand the aft PM magnetic bearings, it should be readily apparent that thenumber of rings used in the PM magnetic bearings can vary withoutstraying from the intended scope of the present invention. Likewise,various directions of magnetization can be used in the PM magneticbearings as is known in the art such as, but not limited to, Halbacharrays. Still further, the magnetic intensity may vary from magnet typeto magnet type and therefore the specific shape and size may vary fromthe specific embodiment shown without straying from the intended scopeof the present invention.

While the fore and aft PM magnetic bearings provide a radial magneticspring force that stabilizes and centers the rotor assembly 60 with apositive spring characteristic, the PM magnetic bearings also create anegative spring characteristic in the axial direction which makes therotor axially unstable. To compensate for the axial negative springcharacteristic, a feedback-controlled voice-coil actuator acts on therotor assembly 60 in the axial direction. It should be understood thatfor applications of the present invention for short-term blood pumpimplants a passive mechanical structure can be used in lieu of a voicecoil such as a jewel bearing or a ball and cap structure to counteraxial forces. However, long term implants would preferably benefit fromthe non-mechanical voice coil arrangement of the preferred embodiment.

The voice-coil actuator is comprised of voice coils 129 a and 129 bwired such that current flows in opposite directions in the two coils129 a, 129 b and thus interacts with magnets 71, 72, and 73 to producean axial force in response to an electronically-controlled current inthe coils 129 a, 129 b. Magnet 68 b also contributes to the function ofthe voice-coil actuator as it is proximal to voice coil 129 a andcontributes to the radial magnetic field in voice coil 129 a. The use ofmagnet 68 b in both the fore PM magnetic bearing and the voice-coilactuator improves electrical power efficiency. The aspect of the presentinvention regarding this integration of the magnetic bearing functionand the voice-coil actuator function is referenced as “incorporating anintegrated PM bearing and voice-coil actuator.”

Feed-back control of the voice-coil actuator in FIG. 5 is accomplishedby using fore and aft position sensor coils 135 and 136. As the rotorassembly 60 moves fore and aft, the impedance of coils 135 and 136change and the impedance change is interpreted as positional change byelectronics external to the blood pump 20. A feedback control algorithmsuch as virtually zero power control (as described by J. Lyman,“Virtually zero powered magnetic suspension,” U.S. Pat. No. 3,860,300,1975 and incorporated herein by reference) is applied to the positionsignal to determine the voltage or current applied to the voice coils129 a and 129 b. The motor windings 126 and motor iron 125 areencapsulated in potting material 127 for structural stability andimproved heat transfer. Similarly, the voice coils 129 a and 129 b areencapsulated in a potting material 130 for the same reasons. Thepositioning the sensor coil 135 at the flow divergence adjacent to theimpeller blades 62 makes use of this space that is not suitable for themotor, PM magnetic bearings, or voice coil actuator. Thus, this designenables a shorter and more compact pump design relative to one where thevolume near the flow divergence is not used. Similarly, the positioningof the sensor coil 136 at the flow convergence adjacent to the statorblades 102 is advantageous for the same reason. This aspect of thepresent invention related to the placement of these sensor coils 135,136 is referenced respectively as “located at a blood flowconvergence/divergence.”

With further regard to FIG. 5, the stator housing 81 extends for a largefraction of the length of the blood pump 20. Stator housing 81 forms theoutside wall of annular flow gap 39 which is a large part of the bloodflow path through the pump. The radial dimension of the flow gap 39 isdenoted Wg as depicted in FIG. 10. Additionally, the stator housing 81supports the stator PM rings 121 a, 121 b, 121 c, 121 d, the pottedvoice coils 129 a, 129 b, the potted motors coils 126 and motor iron125. These components are stacked and clamped in place using stator nut134, collar 132, and end shoulder 93. Spacers may be placed throughoutthe stack to ensure accurate alignment with corresponding components inthe rotor assembly. Collar 132 incorporates a slot (shown as element 133in FIG. 6) enabling the connection of sensor coil 135.

Inflow conduit adaptor 44 a is attached using nut 47 a and a sealbetween the adaptor 44 a and the stator housing 81 is formed with anO-ring 84. O-ring 105 is shown and additional O-ring seals are similarlyindicated in cross-section but are not labeled for clarity ofillustration. While the embodiment as shown uses O-ring seals, it shouldbe readily apparent that welded elements may also be used for producinga compact and sealed housing. Exterior housing cover 23 and housing end26 enclose the components mounted on the stator housing 81. The housingcover 23 and the housing end 26 snap together with snap fit lip 31.Outlet housing 100 incorporates the stationary stator blades 102. Theoutlet housing 100 also includes a stopping face 95 formed at the pointof abutment between the stator housing 81 and the outlet housing 100.The stopping face 95 (seen more clearly in FIG. 8) is narrow relative tothe total cross-section of the outlet housing 100. This relativelynarrow, short dimension allows the stator blades 102 to more easily bemachined from the end of the outlet housing 100. This enables thelow-cost machining of the stator blades 102 and we describe this featureof the pump as having a “short housing end.” The outlet housing 100 isclamped to the stator housing 81 in the axial direction with the tensionin the snap fit lip 31. The outlet conduit adaptor 44 b is clamped tothe outlet housing 100 with nut 47 b.

Also shown in FIG. 5 are the details of the rotor assembly 60. Here, theimpeller housing 61 can be seen to include the impeller blades 62 and adeep cup structure than encloses the various permanent magnets (68 a, 68b, 68 c, 68 d, 70, 71, and 72) used in the motor, voice-coil actuatorand the PM magnetic bearings as well as spacer 75. The impeller housingis capped with a rotor tail 76 that fits on the interior of the cuppedsection of the impeller housing 61 and is held fast with a draw rod 77.Such assembly for enclosing magnets for the motor, magnetic bearings,and voice-coil actuator is referenced as a “draw-rod assembly”. It isunderstood that the rotor tail 76 may additionally be laser-welded tothe interior of the cupped section of the impeller housing 61. All ofthe rotor assembly magnets are mounted on the hub support 74. It shouldbe understood that the fore-to-aft order of the motor and voice-coilactuator can be inverted on the rotor without straying from the intendedscope of the present invention.

The conduit (or cannula) adapters 44 a, 44 b are attached to the inflowend 21 and outflow end 22 of the stator. Each cannula adaptor has aconical tapered end for securing a cannula or graft, a central flangeand a cylindrical portion on the other side adapted for insertion fromthe inflow end and outflow end of the pump. The central flange of eitheradaptor includes a ferromagnetic ring affixed within and to affect amagnetic coupling to the housing when the flange is brought intoco-axial proximity with the pump housing.

The numbered elements shown and described with regard to FIG. 5 arelikewise shown throughout the remaining FIGS. 6 to 10.

FIG. 6 is a section view corresponding to cross-section taken acrossline B-B of FIG. 3 and shows the interface of the cable assembly 53 inrelation to the blood pump 20 via a dome 24 in the exterior housingcover. The cable assembly 53 carries conductors for interconnection (notshown) through cavity 25 to the voice coils 129 a, 129 b, sensor coils135, 136, and the motor windings 126. It should be understood that suchmanner of conductor interconnection is well known within the electricalart and not further described herein. The two voice coils 129 a, 129 bhave four terminations shown—one of which is labeled 131. The motor andsensor coil terminations are not shown, but are similarly configured asis known in the electrical art. In addition, FIG. 6 shows the slot 133in the standoff collar 132 through which the sensor coilinterconnections are made. Similarly, slot 92 is shown in the housingend 100. Additional features of the cable assembly 53 are thefeed-through body 51 which is press-fit into the housing end cap 26, andthe feed-through nut 52. Pin 28 in pinhole 109 locks the relativeorientation of the outlet housing 100 to the housing end cap 26. Screw120 fastens the outlet housing 100 to the stator housing 81. Magnets 69of the four-pole motor are oriented outward, and the diverging face ofthe impeller 65 is indicated.

FIGS. 7 and 8 are respective detail views C and D as indicated in FIG. 5showing the inlet and outlet paths of blood flow. The inlet path of FIG.7 shows the impeller blades 62 detailed relative to the sensor coils 135which are wound in groove 89. Likewise, the outlet path of FIG. 8 showsthe stator blades 102 detailed relative to the sensor coils 136 wound ingroove 108. The sensor coils 135, 136 are of a truncated cone shape asshown in order to maximize their fractional change in impedance due torotor motion. Alternatively, grooves 89 and 108 can be filled completelywith wire with only that part of the coil closest to the rotor assembly60 having a conical shape. In either case, the sensor coils 135, 136 arereferenced as being “cone-shaped.” FIG. 8 shows the draw rod 77, threads78 (not detailed), and thread relief 79 used to assemble and fasten therotor elements. In both FIGS. 7 and 8, the annular flow gap 39 isvisible relative to the inflow lumen 37 and outflow lumen 38. From thisdetail, it should be readily apparent that the annular flow gap 39 is arelatively large gap approximately equivalent to the radius of lumens37, 38.

FIG. 9 is a side view of the stator assembly 80 according to thepreferred embodiment with the exterior housing cover and cannulaadaptors removed. In this view, O-rings 86 and 105 are visible.Additionally, stator nut 134 threaded onto stator housing 81 is shown atthe left side of FIG. 9. The stator nut 134 clamps a stack of componentsbetween itself and the housing 81. The stack is comprised of (from leftto right) collar 132, stator PM rings 121 a and 121 b, voice coilpotting 130 (containing voice coils), motor potting 127 (containingmotor iron and motor windings), and stator PM rings 121 c and 121 d.Outlet housing 100 is also shown. A standoff collar 110 is used toprovide a window for routing conductors from the stator inlet sensorcoils. A slot (not shown) is cut out of the housing 81 at the outflowend (at the right side of FIG. 9) to provide a window for routing theconductors from the stator outlet sensor coils.

FIG. 10 is a section view corresponding to the cross-section takenacross line E-E of FIG. 9. A portion of the impeller blades 62 arevisible in this cross-section within the annular flow gap 39. Also shownare the stator PM ring 121 a, annular flow gap dimension Wg, stator PMring radial width Ws, rotor PM ring 68 a, rotor PM ring radial dimensionWr, rotor outside radius Rr, stator inside radius Sr, stator housing 81,the impeller housing 61, draw rod 77, and hub support 74.

FIG. 11 is a cross-section view taken across line F-F in FIG. 9 showingthe cross-sectional details of the motor assembly 124. The largestdiameter component shown is the motor potting 127. Working inward, thenext adjacent components are the twelve motor windings 126 a to 126 nfor this 3-phase 4-pole motor configuration which are wound in atoroidal fashion around the motor iron 125. A layer of potting materialappears on the inside of the coils and then interior to that is thestator housing and the annular flow gap 39. Interior to the rotor areshown the outwardly magnetized magnets 69 a and 69 b and the inwardlymagnetized magnets 70 a and 70 b which are mounted on the hub support74. Draw rod 77 is at the center of the FIG. 11.

Although the voice-coil magnets 71, 72, and 73 are shown throughout thefigures in a particular magnetization, the reverse magnetizations tothose shown may serve the purpose of creating a magnetic field at thecoils 129 a, 129 b with a radial component that is inward for one of thecoils and outward for the other. Other magnetic assemblies are alsopossible without straying from the intended scope of the presentinvention including, but not limited to, those with ferrous materialssuch as iron.

According to operation of the preferred embodiment, the maglev VADincludes an actively-controlled axial suspension and a passive radialsuspension. PM bearing rings 68 a, 68 b, 68 c, and 68 d near each end ofthe rotor interact with PM bearing rings 121 a, 121 b, 121 c, and 121 don each end of the stator housing 81 to maintain the rotor in a coaxialrelationship with the stator inlet and outlet housing. In simple terms,the PM bearings serve as springs that support the rotor in the radialdirection. This mass-spring system can oscillate in a number of modesand corresponding modal frequencies determined by the stiffnesses of thePM magnetic bearings, the mass and rotational inertias of the rotor, andthe rotor speed (due largely to gyroscopic effects). As the rotor speedvaries, inertial and magnetic imbalances, can cause vibrational motionsof the rotor with the rotor speed is at or near a modal frequency calleda critical speed. These large motions, in turn, can cause the impellerblades 62 or stator blades 102 to rub, respectively, against the statorhousing 81 and/or rotor tail 76. Such rubbing or touchdown isundesirable so that operation near the critical speeds is to be avoided.

By choosing the mass of the rotor sufficiently large, choosing thestiffness of the PM magnetic bearings sufficiently small (largely byincreasing the fluid gap Wg and hence the magnetic gap Sr minus therotor radius Rr in FIG. 10), and choosing the speed range of the rotorto include speeds above at least one critical speed, the inventive bloodpump can be designed of significantly reduced size relative to the priorart. Such a magnetically-levitated blood pump in accordance with thepresent invention whose pumping range includes speeds above at least onecritical speed is called a supercritical maglev pump. The presentinvention is advantageous in that increasing the magnetic gap Sr minusRr enables an increase in the fluid gap Wg and impeller blade heightwhich, in turn, increases the pump flow at a given size. This synergy ofcombination elements of the present invention provides that all criticalspeeds for this rigid rotor configuration are below the operating rangeof the pump. Thus, the critical speeds do not limit the maximum speed ofthe pump and a very large range of operation is possible. Further, thelarge fluid gap and high rotor speeds work together to achieve very highflow rates in a small pump configuration. Ratios of fluid gap to rotordiameter Wg/(2Rr) greater than 1/10 (and as high as 1/5.4) combined withsupercritical operation are achievable.

The miniaturization achieved with a large fluid gap combined withsupercritical operations is revealed in other parameters in the pumpconfiguration as well. The ratio of rotor diameter to cannula diameter(equal to inlet or outlet diameter) can be less than 2. As well, therotor can have an relatively high average density. As part of theoptimized configuration in accordance with the present invention, themagnetic bearing stiffness should be chosen sufficiently high so thatunder accelerations, due to motion of the patient for example, the rotordoes not contact the housing.

One approach to optimizing the inventive configuration for supercriticaloperation is to provide for rotor inertias sufficiently large and/or themagnetic bearing stiffness sufficiently small (e.g., by choosing a largeWg) such that the critical speeds are below the speed range needed toaccomplish the desired flow range. Further, as the pump passes throughthe critical speed, there is often vibration in the rotor so the bladetip clearance and the rotor damping are provided to be sufficientlylarge so as to avoid contact between the blade tips and the housing asthe rotor speed passes through a critical speed. The formal process ofanalysis of rotor critical speeds is accomplished through a matrixvibration equation involving the mass matrix, stiffness matrix, dampingmatrix, and gyroscopic matrix for the system. This matrix vibrationequation is uniquely combined with the motor and pump design in thissystem to achieve high overall pump efficiency. More specifically, thedynamics of the rotor are given by:

m{umlaut over (q)}+(C+G(Ω)){dot over (q)}+K _(q) =f(t)

Where q is a vector of which components are translations and rotationsof the rotor excluding axial translation and rotation about the rotoraxis. That is, q, captures the four expected vibrational motions of therotor—two directions normal to the axis of rotation together with pitchand yaw motion of the rotor. M is corresponding mass matrix modeling themass and rotational inertias, C is the damping matrix, Ω is the rotorangular velocity or speed, G(Ω) is the speed dependent gyroscopicmatrix, K is the rotor stiffness matrix due to the PM magnetic bearings,and f(t) is the rotor forcing due to inertial and magnetic imbalances.During the design process, the damping C is assumed to be small orsimply ignored when solving for the eigenvalues of the homogeneousequation (i.e., f(t) set equal to zero). These speed-dependenteigenvalues determine the critical speeds where large rotor motions canoccur.

The pump magnetic and mechanical designs directly affect the matrixvibration equation. For example, adding rotor mass increases componentsof the mass matrix M, increasing the PM bearing gap generally decreasescomponents of the stiffness matrix, K, and improving the rotormechanical balance and magnetic balance reduces the rotor forcing f(t).If the magnetic materials do not have uniform magnetizations or if themagnetic fields are non-uniform due to mechanical imperfections,disturbance forces can be imposed on the rotor which we refer to asmagnetic imbalance. Further, the gap 39 shared by the PM magneticbearings, motor, voice-coil actuator, and pump is one source of designinteraction. That is, the gap 39 affects the performance of all of thesesubsystems. The overall design of the pump is accomplished through thecomputer optimization of pump efficiency subject to constraints on size,flow rate, and speed relative to the critical speeds. A unique featureof the design optimization is that the motor speed is constrained to begreater than at least one of the critical speeds.

Control of the maglev VAD in accordance with the present invention isconfigured to facilitate continuous levitation of the rotor with minimalpower dissipation. The active axial suspension is accomplished by thethrust coil assembly of the stator maintaining the rotor in a fixedaxial position with respect to the stator inlet and outlet housing. Thisactive suspension relies on a signal provided by the eddy current typesensor coils on the inlet end and outlet end to determine the shift inrotor position in an instant in time. The signal from the inlet sensorcoils and the signal from the outlet signal coils are combined in adifferential fashion to minimize noise and coupling to the thrust coils.The inlet and outlet sensor coils may be single coils or preferablecomprised of two counter-wound coils to further reduce noise andcoupling.

It should be understood that a control system (external to the bloodpump) provides current to the thrust bearing coils and energizes them asappropriate for correcting the rotor position in an instant of time.Depending on the direction of current through the axial thrust coils,this will cause either a forward thrust or backward thrust as themagnetic field caused by the energized thrust coils will interact withthe magnetic thrust bearing elements within the rotor. As alreadydiscussed, the axial thrust coil assembly may include two coils that areencased in a thermally conductive and electrically insulation potting.The motor components of the stator and rotor are also displaced betweenthe radial suspension elements. As already discussed, the stator motorassembly may be a series of windings around a motor iron (assembly offerromagnetic laminations) that is subsequently potted in a thermallyconductive and electrical insulative potting material.

The four-pole motor according to the preferred embodiment provides forsubstantially continuous winding connecting the windings of each polewith a minimal number of soldered or crimped terminations. In suchconfiguration, the motor assembly includes three phases with fourinterconnected windings per phase for a total of 12 windings wrappedaround a torrid motor iron (i.e., laminate assembly). As the motor iscontrolled and the windings are energized, this creates a magnetic fieldthat interacts with motor magnets of the rotor for affecting a torque onthe rotor and providing rotational motion. According to the preferredembodiment, supercritical operation at speeds in the range of 15,000 RPMor more can be expected.

Although the present invention has been described herein with referenceto a particular embodiment, it will be understood that this descriptionis exemplary in nature and is not considered as a limitation on thescope of the invention. The scope and spirit of the present invention istherefore broad as to encompass all novel aspects of the invention takenapart or combined together in various configurations as can beenvisioned in the full context of this disclosure.

1. (canceled)
 2. A blood pump, comprising: an annular flow path definedbetween an inner surface of a housing and an outer surface of a rotor;an inflow end of the flow path providing for entry of blood through aninflow conduit; an outflow end of the flow path providing for exit ofsaid blood through an outflow conduit; and the rotor including an inflowelement having at least one blade, the inflow element forming a flowpath divergence, an outflow element on an outflow end forming a flowpath convergence, and a body extending between the inflow element andthe outflow element, the inflow element, outflow element and bodyconnected to one another by a shaft.
 3. The pump of claim 2, wherein theinflow element is positioned at least partially in the inlet conduit. 4.The pump of claim 3, wherein the at least one blade of the inflowelement is at least partially positioned in the inlet conduit.
 5. Thepump of claim 2, wherein the outflow element is positioned at leastpartially in the outflow conduit.
 6. The pump of claim 1, wherein theflow path divergence directs the blood flow from the inlet conduit tothe annular flow path, and the flow path convergence directs the bloodflow from the annular flow path to the outflow conduit.
 7. The pump ofclaim 1, wherein the annular flow path over the inflow element, body andoutflow element allows for minimal blood damage or thrombus formation.8. The pump of claim 1, wherein a width of the annular flow path isapproximately equivalent to a width of the inflow and outflow conduits.9. A blood pump, comprising: an annular flow path defined between aninner surface of a housing and an outer surface of a rotor; an inflowend of the flow path providing for entry of blood through an inletconduit; an outflow end of the flow path providing for exit of saidblood through an outflow conduit; and the rotor including an inflowelement having at least one blade, the inflow element forming a flowpath divergence, and a body extending from the inflow element, whereinthe annular flow path provides for the blood pump to be adapted forminiaturization and for minimally-invasive implantation.
 10. The pump ofclaim 9, wherein the rotor further comprises an outflow element on anoutflow end forming a flow path convergence.
 11. The pump of claim 10,wherein the inflow element, outflow element and body are connected toone another by a shaft.
 12. The pump of claim 9, wherein the inflowelement and body are connected to one another by a shaft.
 13. The pumpof claim 9, wherein the rotor is adapted for supercritical operation,such supercritical operation including a speed of the rotor of 15,000RPM or more.
 14. The pump of claim 9, wherein a ratio of a width of theannular flow path to a rotor diameter is greater than 1/10.
 15. The pumpof claim 14, wherein said ratio is within a range from 1/10 to 1/5.4.16. The pump of claim 9, wherein the housing includes stationary bladesfor directing said outflow of said blood to the outflow conduit.
 17. Thepump of claim 9, further comprising an outer housing cover forencapsulating a stator, the outer housing having a dome providing spacefor electrical connections to and from the pump.